A galvannealed steel sheet has been used worldwide as a steel sheet for vehicles. For this application, corrosion resistance, coating properties, weldability, powdering and flaking resistance during press forming, are required for the galvannealed steel sheet. The coating layer of the galvannealed steel sheet includes a ζ phase, a δ1 phase, and a Γ·Γ1 phase. Among the above-described requirements, press formability represented by powdering and flaking resistance is dependent on the amounts of the ζ phase and the Γ·Γ1 phase. The powdering resistance is enhanced as the Γ·Γ1 phase is reduced, and the flaking resistance is enhanced as the ζ phase is reduced. Therefore, in order to achieve good press formability, a coating layer mainly consists of the δ1 phase is required.
In order to obtain the coating layer mainly consists of the δ1 phase, the composition (Al concentration) and the temperature of a coating bath, and heating and cooling conditions for alloying need to be optimized depending on the steel components. In usual operations, the Al concentration and the temperature of a coating bath are maintained within the limited ranges, and a heating and cooling pattern is determined depending on the alloying rate of the steel. However, in practice, due to operational conditions in an upstream process (a process before galvannealing) such as hot rolling, the alloying rate may vary in parts even in the same type of steel and even in the same coil. Therefore, each time, an operator finely adjusts the heating and cooling conditions while visually checking the degree of alloying. The testing of powdering and flaking as well as phase analysis of the coating is performed off-line using the representative parts (typically a front portion and/or a tale portion) of a coil after production.
However, by checking coating quality through the off-line testing and analysis, quick feedback to adjust the operational condition may not be achieved. Therefore, for example, when an alloying rate is changed due to a change in steel type, there is a risk of a reduction in yield. In addition, for example, depending on hot rolling conditions, alloying of the front portion of a coil may be slower than alloying of the middle portion. In this case, when the operation is performed according to the alloying condition of the front portion, the middle portion is excessively alloyed, and it may lead to poor powdering resistance of most of the parts in that coil.
In order to prevent these problems beforehand, on-line measurement with high accuracy over the entire length of the coil is effective. A technique employed for this purpose is an on-line X-ray diffraction. An X-ray diffraction is a method for qualitative and quantitative measurement of crystal phases in a coating layer using the diffraction phenomenon which occurs when crystals are irradiated with X rays with good parallelism. In order to use this method for the on-line measurement, there is a need to select diffracted X-rays having a good correlation with the crystal phase thickness. Furthermore, in order to obtain high measurement accuracy, there is a need to select diffracted X-rays having a high intensity within a practical range of diffraction angle (2θ). In Patent Documents 1 and 2, 2θ>80°, which corresponds to the crystal lattice spacing d<1.78 Å when Cr is used as an X-ray target, is disclosed as a practical range where the effect of vibration and heat radiation of the steel sheet is minor, and the change in incident X-ray intensity is small. As described in Patent Documents 2 to 5, d=1.26 Å for a ζ phase (2θ=130° when the target is Cr), d=1.28 Å for a δ1 phase (2θ=127° when the target is Cr), and d=1.22 Å for a Γ·Γ1 phase (2θ=139° when the target is Cr) are widely used in the previous art.
It is also necessary for the on-line measurement to calibrate the X-ray intensity based on the Zn coating weight, and to adjust the peak angle due to the Fe % in the coating. In addition, it is also important to reduce the effect of vibration of a steel sheet.
One can use different calibration curves between X-ray intensity and the thickness of alloy phase based on the Zn coating weight in order to offset the effect of coating weight (Non-Patent Document 1).
On the other hand, as disclosed in Patent Document 6, by measuring a diffracted X-ray intensity IΓ of a Γ phase corresponding to d=1.22 Å and a background intensity IB near a diffracted X-ray position, it is possible to obtain a degree of alloying defined as (IΓ−IB)/IΓ based on a single calibration curve. In this method, it is assumed that since the effect of the coating weight is reflected on IB, the correction can be achieved.
In Patent Document 5, a method of correcting a change in a diffracted X-ray peak angle due to a change in Fe % in a coating layer is disclosed. Fe—Zn alloy phases have Fe % ranges. Γ phase, for example, has a Fe % range of 20 to 28 mass %. Therefore, a crystal lattice spacing changes according to a degree of alloying, and an appropriate diffraction angle 2θ also changes according to the degree of alloying. Patent Document 5 is a technique of, in order to ascertain a change in the diffraction angle 2θ, allowing a detector to scan a range of 2 to 5° from 2θ on an arc path. By using this technique, an appropriate range of alloying conditions can be more accurately determined compared to a case where the detector is fixed.
In Patent Document 7, a technique of reducing the effect of vibration of a steel sheet is disclosed. In Patent Document 7, an incident X-ray beam is introduced into a multi-layer optics to be collimated. As a result, diffracted X-rays generated by irradiating the steel sheet with the incident X-ray beam are collimated. Therefore, even when the distance between the detector and the X-rays diffraction position on the steel sheet changes due to vibration, the intensity of the detected X-rays does not change significantly.